[editor] add PhasmaMCP integration

[christoskaramou]

Summary:

• Vendor PhasmaMCP under third_party and include it in the Spartan build.
• Add an opt-in editor MCP server via --mcp with port/oauth toggles.
• Expose spartan_status and search_codebase tools backed by a source index.

Verification:

• Built debug x64 with MSBuild successfully.
• Launched spartan_vulkan_debug.exe --mcp --mcp-port=8765.
• Verified /health, MCP initialize, tools/list, spartan_status, and search_codebase.